r8169: tweak the PCI data parity error recovery

The 8110SB based n2100 board signals a lot of what ought to be
PCI data parity errors durint operation of the 8169 as target.
Experiment proved that the driver can ignore the error and
process the packet as if nothing had happened.

Let's add an ad-hoc knob to enable users to fix their system while
avoiding the risks of a wholesale change.

Signed-off-by: Francois Romieu <romieu@fr.zoreil.com>